DESCRIPTION We seek to understand in S. cerevisiae the mechanisms that allow the conversion of a shallow gradient of an extracellular mating signal into a discrete polarity decision to produce a cellular projection in the direction of a mate. A screen will be conducted to search for mutants unable to carry out chemotropic matting, by selecting for yeast which can only mate at the site known to be utilized when polarity signals are absent; we will determine the function of these proteins by assessing their localization and binding partners in the absence and presence of pheromone. We also propose novel methods for observing the hypothesized amplification of the external gradient. These studies may lead to improved understanding of leukocyte chemotaxis, which utilizes the same signal transduction cascade.